under sink.

When I had rat problems in our barn many years ago, I used Just One Bite bars. I'm sure there are other products that work just as well, but those are what a farmer friend recommended, and they did the job for me.

However, I strongly recommend against any kind of poison if you have pets or young kids. In that case, the strong-spring hard plastic traps that you set with a squeeze (I don't recall the name) are pretty effective, and are reusable if you're not too squeamish about such matters. Peanut butter is an effective bait.

I have used; spray foam, steel wool, and hardware cloth. Plywood, vinyl flooring, and Hardi board.

For an opening your size, as a DIY, to prevent rodents, I would start with hardware cloth. Cut to fit, staple in place. Re holding up. All of the fixes have a life time. “Spray and forget it”, may not work, and may cause problems with future work. Steel wool may contribute to electrolysis. Maybe hardware cloth allows Air Conditioned Air to vent?

I went with hardware cloth and staples. thank you for all tje suggestions and you time.